1. Field of the Invention
The present invention relates to an optical communication system that uses a plastic optical fiber as the transmission line.
2. Description of the Related Art
In optical communication systems, single mode or multimode optical fibers made mainly of silica glass have been used as the optical transmission lines. In general, the core diameter of these optical fibers is less than 100 μm, and very precise core alignment with an allowance of few micrometers is required when splicing the fibers. Consequently, installation work for optical fibers under common environments, including construction sites or the like, is no easy task. This has been an obstacle that lies in the way toward wider use of optical communication systems using optical fibers.
In the mean time, plastic optical fibers having larger core diameters for use with optical communication systems have also been developed as described, for example, in U.S. Patent Application Publication No. 20050232537. Most of these fibers, however, have step index structures due to manufacturing constraints, making them difficult to transmit signals at high speeds over a long distance. Recently, however, a graded index type plastic optical fiber (GI-POF) that has a large core diameter, yet without these drawbacks described above and is applicable to high speed transmission has proposed, and is currently under development for practical applications.
Further, there is a growing demand in the market for an optical communication system capable of transmitting both high and low speed signals. Consequently, a communication system having two transmitters is proposed. One of the transmitters is equipped with a semiconductor laser for high speed signal transmission, and the other is equipped with an inexpensive RC-LED (Resonant Cavity Light Emitting Diode) for low speed signal transmission. The system uses a GI-POF applicable to both high and low speed signal transmission as the transmission line, and either of the transmitters is selected according to the speed of the signal to be transmitted.
As for the plastic fibers applicable to both high and low speed signal transmission, medium diameter GI-POFs with a core diameter in the range from 300 to 600 μm and a numerical aperture in the range around from 0.2 to 0.4 have been developed. Hereinafter, the medium diameter GI-POF will be described in detail.
Basically, such a GI-POF includes a core with a diameter in the range from 300 to 600 μm, and a clad with a thickness of several tens of micrometers surrounding the core. The refractive index of the core is slightly higher than that of the clad in order to confine the propagating light within the core as in an ordinary optical waveguide. A low molecular weight compound having a high refractive index with a large molecular volume is added to the core as a refractive index adjuster. The compound is distributed in the radial direction with the center of the core having a highest refractive index. The refractive index profile “g” within the core is adjusted to a value in the range from 1 to 5. The numerical aperture (NA) of the optical fiber may be defined by the following formula using the maximum refractive index N1 of the core and the refractive index N2 of the clad. In the medium diameter GI-POF, the difference ΔN {=(N1+N2)/N1} is adjusted so that the value of the NA falls in the range from 0.2 to 0.4.NA=N1√{square root over (2ΔN)}
In the mean time, any material may be used for the clad as long as it has favorable characteristics for optical transmission, such as providing a lower refractive index than that of the core and the like. In particular, however, use of fluorinated polymers is preferable from the view point of refractive index. More specifically, polyvinylidene fluoride (PVDF, N2=1.42 for wavelength at 650 nm) may be listed as one of such materials. Likewise, as for the material of the core, any material may be used as long as it has favorable characteristics for optical transmission. In particular, however, polymers having high optical transparency are preferable. Here, use of amorphous polymers is preferable in order to prevent optical anisotropy from developing or to reduce it. Further, use of polymers having superior adhesiveness with each other is preferable for the core and clad. Still further, use of polymers having superior mechanical and thermal resistance properties is more preferable.
Specific material examples of the core may include (meta) acrylic esters (fluorine-free (meta) acrylic ester, fluorine containing (meta) acrylic ester), styrene compounds, vinylesters, polymerized compounds produced using bisphenol A, which is a raw material of polycarbonates and the like as polymerizable compounds. Further, homopolymers produced by using these materials as raw materials and polymerizing each of the materials, copolymers produced by combining two or more of the materials in various ways and polymerizing them, or mixtures produced by combining the honopolymers and copolymers in various ways may be used as the material of the core. Of these, (meta) acrylic esters or fluorine polymer based materials are particularly preferable to form an optical transmission body. In the mean time, if an optical element is used for near-infrared applications (e.g., light source, transmission, or the like at a wavelength in the range from 750 to 850 nm), an absorption loss occurs due to C—H binding forming the optical element. The wavelength region where the absorption loss occurs may be shifted to a longer wavelength region by the use of a polymer in which hydrogen atoms in C—H binding are substituted by deuterium atoms, fluorine atoms, or the like as described, for example, in U.S. Pat. No. 5,541,247 and Japanese Unexamined Patent Publication No. 2003-192708. In this way, the transmission loss of the light transmitted through the optical fiber may be reduced. Such polymers include deuterated polymethyl methacrylate (PMMA-d8), polytrifluoroethyl methacrylate (P3FMA), polyhexafluoro isopropyl 2-fluoroacrylate (HFIP 2-FA), and the like. Preferably, impurities and foreign substances that may act as light scattering sources are removed sufficiently from the raw material compound prior to polymerization in order to ensure the transparency of the material after polymerization.
In the mean time, it has been revealed that when an optical signal is transmitted through the medium diameter plastic optical fiber with a core diameter in the range from 300 to 600 μm as described above by the optical communication system using the transmitter equipped with a RC-LED for low speed transmission, the transmitted optical signal may be attenuated by the optical fiber greater than expected. More specifically, as shown in FIG. 7, light L emitted from a light source constituted by a RC-LED 2 having a luminous section 2 with a diameter of 100 μm enclosed by a transparent resin 4 having a lens-like convex section 3 was inputted to an optical fiber (GI-POF) with a core diameter of 500 μm and transmitted through the fiber, an unexpectedly large amount of transmission loss exceeding 250 dB/km was measured.
In view of the circumstances described above, it is an object of the present invention to provide an optical communication system that uses a medium diameter plastic optical fiber, such as the GI-POF or the like, yet capable of achieving low transmission loss.